/Engine/Source/Runtime/Engine/Classes/Animation/AnimMontage.h
Check if this slot has a valid additive animation for the specified slot. The slot name should not include the group name. i.e. for "DefaultGroup.DefaultSlot", the slot name is "DefaultSlot".
bool IsValidAdditiveSlot ( const [FName](API\Runtime\Core\UObject\FName) & SlotNodeName ) const